The present invention relates to an image processing system for printing, in a laser printer, a composite image which contains a multi gradation-level image, for example, a photograph and a monotone image (e.g., characters or a line drawing) in a superposed manner.
In the laser printer, the width of a dot to be formed on a paper can readily be varied by adjusting the width of a laser pulse. To print a multi gradation-level image such as a photograph, with a laser printer, the image is converted to corresponding image data. In the conversion to image data, each pixel of the image is virtually divided into small rectangles. A gradation level or value of each pixel is expressed by the number of small rectangles that are to be painted black with a laser beam or in which dots are to be formed (viz., how the laser pulse width is selected for the pixel). If one pixel is divided into N small rectangles, for example, a gradation value of one pixel may be expressed in (N+1) steps ranging from 0 to N. The multi gradation-level image thus converted will be hereinafter referred to as an "N-fold density image."
The N-fold density image consists of binary data trains, in which binary data each consisting of a given number of bits, which are representative of a pattern-of-dot array in each pixel, are arranged in accordance with an array of pixels. Various methods for reducing the number of bits of the binary data of each pixel have been known. The binary data of the pixels, which form an N-fold density image, indicates the small rectangle or rectangles to be painted black with a laser beam, viz., in which a dot or dots are to be formed. Accordingly, the printer can print the multi gradation-level image on a paper by modulating the laser pulse width in accordance with the binary data.
The dots printed by the laser printer have such a nature that closely located dots likely gather under the electric field, to form a dot block of which the configuration is smooth. When a monotone (black/white) image of characters or a line drawing is printed by the laser printer, a technique of smoothing is frequently used. Smoothing techniques make the contours of characters or a line drawing smooth and natural by taking advantage of the above nature of closely located dots. Basically, the monotone image is expressed by binary data trains in which one bit, assigned to one pixel, indicates whether or not the pixel is black or white (a dot is formed or not). In the smoothing process, small dots corresponding to the small rectangles are added to or removed from the pixels located on the contour of a character or a line drawing (both will be referred to as a character) in the monotone image. In the smoothing process, the monotone image is expressed by binary data trains in which a plural number of bits are assigned to one pixel as in the N-fold density image.
As seen from the above description, the multi gradation-level image is faithfully printed in accordance with its binary data train to maintain the accuracy of the printed picture. In the monotone image, its binary data trains are subjected to the smoothing process to enhance the readability and clarity of the printed picture. Therefore, the necessity of the smoothing depends on whether or not the image data input to the printer is a monotone image or a multi gradation-level image.
An image in which characters or a graph is half-tone screened is equivalent to a composite image in which a monotone image of characters or a graph and a multi gradation-level image of the half-tone screen are superposed one on the other. In this case, it is impossible to obtain the best print result, regardless of whether the smoothing is used. The reason for this is that if the smoothing is performed, the characters of the monotone image will be corrected to have beautiful contours. However, the smoothing would also be applied in part to an area of the multi gradation-level image. The result is generation of unwanted noise particularly in the halftone region of the image. If smoothing is not performed, a region of the multi gradation-level image is finely printed, but the contours of the characters of the monotone image are not likely to be smooth.